Had a much longer reply mostly done, then my computer decided to take a nap and all that typing went poof; so I'll boil it down to this:
Rules-as-physics (or, perhaps, physics as rules) provide a baseline underpinning that allows a character to try anything, whether "within" the game rules or not, with a vague sense of what might happen next.
Gravity is an unwritten game rule because we already know how it works, reflected by jumping and falling rules etc. The ways in which magic interacts with and affects other in-game physics (as in, at the base level) should be written game rules such that we all know how it works and can extrapolate and-or rationalize other game rules on top of it cf gravity vs jumping.
Rules-as-physics doesn't need - or mean - a rule to cover every possibility. In fact, the opposite is almost the case: if the physics-as-rules are nailed down the DM is in a far better position to make consistent calls when players try things the game rules don't already cover.